In recent years, prevalence of portable terminals such as a portable phone, a notebook type personal computer, and a pad type information terminal apparatus is considerable. A lithium ion secondary battery is used as a secondary battery that is used as a power source for these portable terminals. Since a more comfortable portability is demanded in the portable terminals, scale reduction, thickness reduction, weight reduction, and performance enhancement of the portable terminals are rapidly developing, and the portable terminals are coming to be used in various fields. This trend is currently still continuing, and further scale reduction, thickness reduction, weight reduction, and performance enhancement of batteries used in the portable terminals are also demanded.
A non aqueous electrolyte battery such as a lithium ion secondary battery has a structure such that a positive electrode and a negative electrode are disposed via a separator and housed in a container together with an electrolytic solution obtained by dissolving a lithium salt such as LiPF6, LiBF4, LiTFSI (lithium (bistrifluoromethylsulfonylimide)), or LiFSI (lithium (bisfluorosulfonylimide)) into an organic liquid such as ethylene carbonate.
Typically, the negative electrode and the positive electrode are formed in such a manner that a slurry for electrodes, which is obtained by dissolving or dispersing a binder and a thickening agent into water and mixing the resulting material with an active material, an electroconductive auxiliary agent (electroconductivity imparting agent) that is added in accordance with the needs, and the like (the slurry for electrodes may hereafter simply referred to as a slurry), is applied onto a current collector, followed by drying water to bind as a mixed layer. More specifically, for example, the negative electrode is formed in such a manner that a carbonaceous material serving as an active material capable of intercalating and deintercalating lithium ions, acetylene black serving as an electroconductive auxiliary agent added in accordance with the needs, and the like are bound with each other onto a current collector such as copper with use of a binder for secondary battery electrodes. On the other hand, the positive electrode is formed in such a manner that LiCoO2 or the like serving as an active material, an electroconductive auxiliary agent that is similar to that of the negative electrode and added in accordance with the needs, and the like are bound with each other onto a current collector such as aluminum with use of a binder for secondary battery electrodes.
Hereto, diene-based rubbers such as styrene-butadiene rubber and acrylic series such as polyacrylic acid are used as a binder for an aqueous medium (for example, Patent Literatures 1 and 2). Examples of the thickening agent include methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, hydroxypropoxycellulose, carboxymethyl cellulose.sodium salt (CMC—Na), and sodium polyacrylate. Among these, CMC—Na is often used (for example, Patent Literature 3).
However, diene-based rubbers such as styrene-butadiene rubber have a low adhesion property to a metal current collector electrode such as copper, raising a problem in that the amount of use cannot be reduced in order to enhance the close adhesion between the current collector electrode and the electrode material. Also, there is a problem in that the diene-based rubbers are vulnerable to the heat generated at the time of charging/discharging, and have a low capacity retention ratio. On the other hand, sodium polyacrylate exhibits a higher adhesion property than the styrene-butadiene rubber series; however, there is a problem in that the electric resistance is high, and also the electrode becomes hard to lack in toughness, so that the electrode is liable to be split. Recently, there is an increasing demand for extension of the time for use of portable terminals and reduction of the charging time, and accordingly, capacity increase (reduction of resistance) of the battery, and improvement in the lifetime (cycle characteristics) and charging speed (rate characteristics) are becoming imperative, so that this is particularly a hindrance.
In non aqueous electrolyte batteries, the battery capacity is influenced by the amount of the active material, so that, in order to increase the amount of the active material in a limited space of the battery, it is effective to suppress the amount of the binder and the thickening agent. Also, the rate characteristics are influenced by the easiness of electron movement, so that it is effective to suppress the amount of the binder and the thickening agent that are non-electroconductive and hinder the movement of the electrons. However, when the amount of the binder and the thickening agent is reduced, the binding property between the current collector electrode and the electrode material and between the active materials in the electrode decreases, so that the durability (battery life) against use for a long period of time considerably decreases, and moreover, the electrode becomes brittle. In this manner, it has been so far difficult to achieve an improvement in the battery characteristics such as battery capacity while keeping the binding property between the current collector electrode and the electrode material and keeping the toughness as the electrode.
The present invention has been made in view of the aforementioned circumstances, and an object thereof is to improve the battery characteristics in the non aqueous electrolyte batteries without deteriorating the function as the binder, that is, the binding property between the active materials and to the current collector electrode and the toughness as the electrode.